Some people might find it odd to build Nasus this way. However with this build you'll become extremily powerful endgame. If an enemy stays inside the spirit fire for it's full duration, you can often do 1000 damage just from that spell. Then you'll just have to slow with "w" and hit them with Siphoning Strike and they won't have a chance. During the game he'll also be extremly good at clearing minion waves and could easily hold a line by himself, while a teammate is jungling etc.

Since the build won't make Nasus OP until the last parts of the game, it's of great importance that you play him right until then. First of you should have a team that fits him well. It could be a good idea to have a jungler and let Nasus solo top. You will get great benefits with this. You can clear minions waves easily allready at low level and you can get your last hits with Siphoning Strike to increase its damage. Just remember that you must recall imediately when you can afford Tear of the Godess and later Rod of Ages. Since those gets better over time. Let the jungler hold your lane for this brief time, in order for you to get max cap at both those items as fast as possible.

When farming, try to target as many minions as possible with Spirit Fire, wait for them to get low hp and then last hit one of them with Siphoning Strike. Remember that it sometimes is better to just basic attack minions or monsters to kill them more slowly and by doing so get more damage on Siphoning Stike.

Always do spells on minions and of course champions. Even if a minion will die of Spirit Fire, do Sophoning Stike to try to get even more Mana and also damage for Siphoning Strike. When you have recalled, remember to activate both nontarget spells as many times you can while your health and mana regenerates and your buying in shop. This way you can reach the max cap a little faster. When you are running back to your lane, activate Siphoning Stike once or twice and let the time when you can strike pass. You don't need to hit anything to get 4 mana, it's enough to just click the abbility.
The reason why you shall finish buying Archangels staff quite late, is because it will be more effective to buy other items before this one, since it will instantly give much abbility power if you buy it late, but if you buy it early it will just give a little more and more over time. If you have Tear of the Godess at max cap before you have bought everything I've put before Archangels staff I recommend you to buy it. I think the reason why is obvious, so I won't even explain that.